1. Field of the Invention
The invention pertains to a magnetic deflector for a trichromatic tube protected by a shield, with in-line guns and a round-luminophor type screen, and to a method for setting a deflector of this type.
2. Description of the Prior Art
A trichromatic or color television tube is a cathode-ray tube with three parts: a relatively flat front side or screen, a cylindrical rear part or neck and a central flared part connecting the screen to the neck. On the screen, each color pixel has three juxtaposed primary luminophor elements of red, green and blue, which are small enough (dimensions of less than 1 millimeter) for the eye not to separate them and for it to receive receive the sum of the three primary light fluxes as a whole. Color is thus achieved by additive synthesis. The neck, which forms the extension of the flared central part, supports three electron guns that project their electronic beams on the screen of the tube so as to excite each luminophor of a particular color. The action of the electron beams is made selective by a perforated mask, placed near the screen: each hole of this mask cuts out a calibrated cylinder of electrons from the beams. The relative angle of convergence of the three beams ensures that the three cylinders are separated, and the depositing of the round luminophors on the screen is such that beam can fall only on those luminophors for which it is intended.
To create a color image on the screen, the entire surface of the screen has to be illuminated by all three beams and, for this purpose, the screen is scanned in successive lines of luminophors using a magnetic deflector. This magnetic deflector comprises coils through which there flow currents of an intensity that may vary according to the angular deflection of the beam to be obtained. One of the coils of the deflector is used to displace the beams horizontally: this is known as line scanning. The other coil is used to displace the beams vertically: this is frame scanning. In conventional color television tubes, the scanning frequencies are 15,625 hertz for the lines and 50 hertz for the frames.
To deposit luminophors when manufacturing the tube, the screen is illuminated by one and the same light source, through the mask, at different angles of incidence which correspond to the selective convergence angles of the three electronic beams so as to define the position of the luminophors on the screen. These three different illuminations of the screen are obtained by an optic lens which gives a precise reproduction, on the light beams, of the deflections which will be obtained by the magnetic deflector on each of the electron beams projected by the electron guns. This shows that the type of magnetic deflector which will be used affects the manufacture of the tube through the optic lens, and this means that a tube manufactured for a certain type of deflector does not work properly if it is mounted with a deflector of another type.
The quality of a trichromatic tube can be defined by three parameters. These are firstly its purity, namely the selectivity of the electron beams with regard to the luminophors, secondly its convergence, namely the convergence of the three beams at one and the same point, and thirdly its sensitivity which is measured as being the energy needed to scan a horizontal or vertical axis of the tube.
It can be seen that if an electronic beam corresponding to a particular color, red for example, excites all or a part of a luminophor of another color, blue for example, the result will be a color that does not correspond to the color sought, and it will be then said that the tube does not have a good degree of purity. A purity defect of this type can be measured by the position of a point, known as the centre of purity, which is the virtual point from where the beam appears to come according to the laws of geometrical optics. Thus when the beam is centered on the luminophor, it seems to come from a center of ideal purity. This center of purity is shifted when the beam is not centered on the luminophor and this shift can take place in three orthogonal directions, namely the horizontal (line) and vertical (frame) directions and the axis of the tube.
As regards convergence, it must first of all be noted that the three electron beams come from three electron guns which are set side by side in a horizontal plane (hence the term "in-line guns") and are separated from one another by a distance of several millimeters. Owing to this arrangement of the electron guns, the corresponding electron beams do not tend to converge on one and the same pixel of the screen in which the distances between the centers of the luminophors are smaller than 1 millimeter. The deflector is provided to correct this defect which is measured by a three-parameter function known as the trilemma T, such that: EQU T=C 3/9+Trh-C 6/12,
where:
C 3/9 is the "3:00 o'clock/9:00 o'clock" (6H/9H) convergence between red and blue. PA1 C 6/12 is the "6:00 o'clock/9:00 o'clock" (6H/12H) convergence between red and blue. PA1 Trh is the horizontal red/blue trapezoid.
Of course, it is sought to obtain T=0, which corresponds to a perfect convergence of the "red" and "blue" beams which are the extreme beams on either side of the central "green" beam.
The above explanations help in understanding the fact that the trichromatic tube is sensitive to modifications of the magnetic field because these modifications affect its purity and convergence characteristics. Consequently, for special applications such as equipment on aircraft or ships, the trichromatic tube should be shielded against unwanted radiation and against the earth's magnetic field by suitable shielding. A shieldig of this type, set around the tube and the magnetic deflector, profoundly modifies the purity, convergence and sensitivity characteristics of the tube because it has an effect on the geometry of the force lines of the magnetic fields created by the deflector coils. The modifications differ according to the type of deflector used.
For a magnetic deflector of the saddle-torus type, in which the saddle-shaped coil is used for line scanning, and the toroid-shaped coil arranged around the line coil is used for frame scanning, the shield has two effects on purity: firstly, it shifts the center of purity towards the rear of the tube along the axis and, secondly, it modifies the position of the line and frame centers of purity. This modification gives an unacceptable level of purity in the tube. The modification of the position of the line and frame centers of purity, i.e. the modification of their coincidence, is due chiefly to the fact that the shield short-circuits the force lines of the magnetic field external to the toroid-shaped frame coil.
The shield also modifies the convergence of the electron beams by increasing the trilemma T which may reach 0.8 millimeters. More precisely, the convergence C 3/9 remains unchanged but the convergence C 6/12 changes from 0 to 0.5 millimeters while the horizontal trapezoid Trh changes from 0 to -0.3 millimeters.
The fact that the shield short-circuits the force lines of the magnetic field also means that a part of the electrical energy applied to the line and frame coils is not used for the scanning. A result of this, there is a loss in sensitivity. Losses of 7% have been measured for line scanning and of 38% for frame scanning.
For a saddle-saddle type of magnetic deflector, in which both coils (line and frame) are saddle-shaped, with the frame coil surrounding the line coil, the effects of the shield on the characteristics of the trichromatic tube are smaller than those encountered in a saddle-torus deflector. Thus, the corvergence measured by the trilemma T can be adjusted along the entire surface of the screen at a value compatible with the requisite quality while the sensitivity is not modified. By contrast, it has been observed that the shield affects the purity along the vertical axis 6H/12H, especially in the corners of the screen where the error reaches half a luminophor.
The above comparison between these two types of magnetic deflector in the presence of a shield shows that a saddle-saddle type of deflector is more appropriate for use with a shield. However, it must be noted that there are special problems to be resolved in high-definition trichromatic tubes which have line scanning at a high frequency of about 64 kilohertz. For the pitch of the mask holes is at least twice or three times smaller than the pitch for holes in masks of conventional trichromatic tubes, entailing severer requirements in terms of purity and convergence.
Another problem to be resolved pertains to the correction of the so-called coma error, which is due to the fact that the electron beams are not subjected to the same magnetic field in the deflector and are therefore deflected differently. In conventional trichromatic tubes this error is corrected, before entry into the deflector, by means of magnetic parts set on either side of the red and blue beams: the purpose of these magnetic parts is to short-circuit the force lines of the magnetic field and, hence, to modify the magnetic field and the path of the beam. These magnetic parts cannot be used for a high scanning frequency because they get heated, a fact that modifies their effect on the beams.